This invention relates to a process for processing natural gas to produce liquefied natural gas (LNG) that has a high methane purity. In particular, this invention is well suited to co-production of LNG by integration into natural gas processing plants that recover natural gas liquids (NGL) and/or liquefied petroleum gas (LPG) using a cryogenic process.
Natural gas is typically recovered from wells drilled into underground reservoirs. It usually has a major proportion of methane, i.e., methane comprises at least 50 mole percent of the gas. Depending on the particular underground reservoir, the natural gas also contains relatively lesser amounts of heavier hydrocarbons such as ethane, propane, butanes, pentanes and the like, as well as water, hydrogen, nitrogen, carbon dioxide, and other gases.
Most natural gas is handled in gaseous form. The most common means for transporting natural gas from the wellhead to gas processing plants and thence to the natural gas consumers is in high pressure gas transmission pipelines. In a number of circumstances, however, it has been found necessary and/or desirable to liquefy the natural gas either for transport or for use. In remote locations, for instance, there is often no pipeline infrastructure that would allow for convenient transportation of the natural gas to market. In such cases, the much lower specific volume of LNG relative to natural gas in the gaseous state can greatly reduce transportation costs by allowing delivery of the LNG using cargo ships and transport trucks.
Another circumstance that favors the liquefaction of natural gas is for its use as a motor vehicle fuel. In large metropolitan areas, there are fleets of buses, taxi cabs, and trucks that could be powered by LNG if there were an economic source of LNG available. Such LNG-fueled vehicles produce considerably less air pollution due to the clean-burning nature of natural gas when compared to similar vehicles powered by gasoline and diesel engines which combust higher molecular weight hydrocarbons. In addition, if the LNG is of high purity (i.e., with a methane purity of 95 mole percent or higher), the amount of carbon dioxide (a xe2x80x9cgreenhouse gasxe2x80x9d) produced is considerably less due to the lower carbon:hydrogen ratio for methane compared to all other hydrocarbon fuels.
The present invention is generally concerned with the liquefaction of natural gas as a co-product in a cryogenic gas processing plant that also produces natural gas liquids (NGL) such as ethane, propane, butanes, and heavier hydrocarbon components. A typical analysis of a natural gas stream to be processed in accordance with this invention would be, in approximate mole percent, 92.6% methane, 4.7% ethane and other C2 components, 1.0% propane and other C3 components, 0.2% iso-butane, 0.2% normal butane, 0.1% pentanes plus, with the balance made up of nitrogen and carbon dioxide. Sulfur containing gases are also sometimes present.
There are a number of methods known for liquefying natural gas. For instance, see Finn, Adrian J., Grant L. Johnson, and Terry R. Tomlinson, xe2x80x9cLNG Technology for Offshore and Mid-Scale Plantsxe2x80x9d, Proceedings of the Seventy-Ninth Annual Convention of the Gas Processors Association, pp. 429-450, Atlanta, Ga., Mar. 13-15, 2000 for a survey of a number of such processes. U.S. Pat. Nos. 5,363,655; 5,600,969; and 5,615,561 also describe relevant processes. These methods generally include steps in which the natural gas is purified (by removing water and troublesome compounds such as carbon dioxide and sulfur compounds), cooled, condensed, and expanded. Cooling and condensation of the natural gas can be accomplished in many different manners. xe2x80x9cCascade refrigerationxe2x80x9d employs heat exchange of the natural gas with several refrigerants having successively lower boiling points, such as propane, ethane, and methane. As an alternative, this heat exchange can be accomplished using a single refrigerant by evaporating the refrigerant at several different pressure levels. xe2x80x9cMulti-component refrigerationxe2x80x9d employs heat exchange of the natural gas with a single refrigerant fluid composed of several refrigerant components in lieu of multiple single-component refrigerants. Expansion of the natural gas can be accomplished both isenthalpically (using Joule-Thomson expansion, for instance) and isentropically (using a work-expansion turbine, for instance).
While any of these methods could be employed to produce vehicular grade LNG, the capital and operating costs associated with these methods have generally made the installation of such facilities uneconomical. For instance, the purification steps required to remove water, carbon dioxide, sulfur compounds, etc. from the natural gas prior to liquefaction represent considerable capital and operating costs in such facilities, as do the drivers for the refrigeration cycles employed. This has led the inventors to investigate the feasibility of integrating LNG production into cryogemc gas processing plants used to recover NGL from natural gas. Such an integrated LNG production method would eliminate the need for separate gas purification facilities and gas compression drivers. Further, the potential for integrating the cooling/condensation for the LNG liquefaction with the process cooling required for NGL recovery could lead to significant efficiency improvements in the LNG liquefaction method.
In accordance with the present invention, it has been found that LNG with a methane purity in excess of 99 percent can be co-produced from a cryogenic NGL recovery plant without increasing its energy requirements and without reducing the NGL recovery level. The present invention, although applicable at lower pressures and warmer temperatures, is particularly advantageous when processing feed gases in the range of 400 to 1500 psia [2,758 to 10,342 kPa(a)] or higher under conditions requiring NGL recovery column overhead temperatures of xe2x88x9250xc2x0 F. [xe2x88x9246xc2x0 C.] or colder.